This is unfortunately not how neutrons behave. Almost everything is nothing but space and so neutrons are quite penetrating since the don’t see the electron clouds. Then there is the prompt gammas and activation. Its not really a big deal, but it is something that will part of standard operational procedures. Water is indeed a good neutron absorber if you have a lot of it (needs lots of volume or they diffuse out). Most is absorb by hydrogen to become deuterium. The Boron will be quite dilute in gas form and will be practically transparent to neutrons.
